In the hot rolling of steel and other metals, it is very common to use high pressure water jets to remove the scale which forms on the surface of the material, in particular in plate and Steckel Mills, or hot strip mills, but descaling may be required in other types of mill.
Most high pressure water descaling systems use flat fan shaped jets as illustrated in FIGS. 1A and 1B. FIG. 1A shows a side view. A header 1 supplies water through a nozzle 2 as a spray 6 to a surface 3 of a plate to be descaled, which is moving in the direction of the arrow 4. A nozzle tip 5 is positioned at a standoff distance h2 above the surface 3 and has an angle of inclination of the nozzle from the vertical β. The angle of inclination is intended to prevent the high pressure water and scale bouncing back from the surface of the slab from interfering with the direct jet from the nozzle tip. FIG. 1B illustrates this seen from end on. The header 1 has multiple nozzles 2, separated by a distance E. Across the width of the plate or material, the spray 6 extends over a spray angle α. Adjacent sprays 6 across the width overlap by an amount D. Seen from above, each spray is offset by an offset angle γ relative to a line across the width of the plate, perpendicular to the direction of movement. The offset angle is intended to prevent neighboring jets from interfering with each other.
One problem with using these flat fan shaped jets is that the overlap area 7 and distance D between adjacent jets 6a, 6b produced by each nozzle is very critical for the performance of the descaling. This is illustrated in FIGS. 2 and 3. If D is too big, i.e. there is too much overlap between the jets, as illustrated in FIG. 2, then water flow 8 on the surface 3 of the material which is created by the leading jet 6a in the overlap region 7 gets in the way of the jet 6b from the ‘following’ jet in the overlap region and reduces the impact of this following jet on the material in the overlap region 7 which can result in stripes with poor descaling on the surface of the material. This phenomenon is described in FIG. 6 and the associated text of the article “Audits of Existing Hydromechanical Descaling Systems in Hot Rolling Mills as a Method to Enhance Product Quality: Juergen W. Frick, Lechler GmbH”. If the overlap D is too small, or even negative, i.e. there is a gap between adjacent jets 6a, 6b as shown in FIG. 3, then the material is not descaled properly and this also produces stripes with poor descaling. This phenomenon is also described in the Audits article mentioned above in FIG. 6 and the associated text.